In the conventional die-casting process, the liquid metal is usually forced into the cavity at such a high speed that the flow becomes turbulent or even atomized. As a result, air is often trapped within the cavity, leading to high porosity in the part which reduces the part strength and can cause part rejection if holes appear on the surface after machining. Moreover, parts with high porosity are unacceptable because they usually are not heat-treatable, thus limiting their potential applications; further, voids can alter the natural frequency of the parts randomly, thus yielding unpredictable vibrational and/or acoustic performances.
Intuitively, the porosity due to turbulent or atomized flow could be eliminated if the viscosity of the metal flow could be increased to reduce the Reynolds number sufficiently so that laminar flow could be produced and the amount of trapped air be minimized, somewhat similar to the injection molding of plastics. However, it was not clear how this could be achieved until the early 1970s when Metz and Flemings proposed the concept of semi-solid material (SSM) processing (Metz, S. A. and Flemings, M. C., "A Fundamental Study of Hot Tearing", AFS Transactions, vol 78, pp.453-460 1970!). They suggested that, if metal solidification is carded out in semi-solid state, the maximum die temperature can be reduced significantly.
The pioneering study of Spencer et al. (Spencer, D. B., Mehrabian, R., and Flemings, M. C., "Rheological Behavior of Sn-15% Pb in the Crystallization Range," Metallurgical Transactions, vol. 3, pp. 1925-1932 1972!) showed that when molten metal is agitated during cooling below its liquidus temperature, the dendritic primary solids will be broken into near-spherical particles suspended in the liquid metal matrix. The viscosity of such a semi-solid slurry increases exponentially with the solid fraction, and it exhibits a shear thinning behavior.
Some die-casting-like manufacturing processes for SSM have been proposed in the past two decades, as shown in Table 1.
Flemings, et. at., received a number of patents on this process, including U.S. Pat. No. 3,902,544, "Continuous Process for Forming an Alloy Containing Non-Dendritic Primary Solids", issued in 1975. This patent is primarily directed at the production of the alloy, although it is indicated that the resulting metal will be removed from the agitation zone and cast.
Flemings et al. (Flemings, M. C., Riek, R. G. and Young, K. P., "Rheocasting", Materials Science and Engineering, vol. 25, pp.103-117 1976! prepared SSM slurry separately and poured it into the shot chamber of a die-casting machine (Rheocasting), where the SSM was injected into the die cavity by a plunger.
Tissier et at. (Tissier, A., Apelian, D., and Regazzoni, G., "Magnesium Rheocasting: A study of Processing-Microstructure Interactions", Journal of Materials Science, vol.25, no.2B, pp.1184-1196 1990!) reported that porosity could still be high in rheocasting since the semi-solid slurry is exposed to the air during stirring at low solid fraction. Furthermore, the plunger mechanism of a die-casting machine does not provide appropriate agitation which is necessary to prevent the formation of a dendritic skeleton, and the high injection speed can introduce mixing of the air with the material in the chamber.
Thixocasting (Flemings et at., "Rheocasting", cited above 8 1976!) is a modification of the Rheocasting process; the material is first rheocast as a billet, cut to appropriately sized slugs and then remelted back to the solid-liquid state for die casting. However, Thixocasting is a two-step process and requires feed materials to be prepared in a separate process, making the operation more costly because of the high cost and low availability of premium billets or powders for SSM processing.
Thixomolding is a different approach where magnesium pellets or particles are fed into a screw injection machine where the chips are convened into SSM slurries by heating and shearing (Bradley, N. L., Wieland, R. D, Schafer, W. J., and Niemi, A. N., U.S. Pat. No. 5,040,589, 1991). However, although porosity might be reduced compared to pressure die casting, it cannot be eliminated and will still be a problem because air (or inert gas) will enter the barrel with the pellets and become a source of porosity in the part. Also, the feed material must be in chip or granular form; thus, if the raw material is in the form of a bar, plate or ingot, a pre-process cutting step is required. Excessive wearing may also occur since the screw is in direct contact with the solid pellets near the feed throat.
Pryor, et. al., U.S. Pat. No. 4,537,242 (1985), presents a "Method and Apparatus for Forming a Thixoforged Copper Base Alloy Cartridge Casing". Like the other Thixo-processes, the SSM is first formed, then cast (solidified) and heated to remelt it in the casting process.
Hirai, et.al., U.S. Pat. No. 5,144,998 (1992) is for a "Process for the Production of Semi-Solidified Metal Composition," Hirai is primarily directed to controlling the solid fraction of the resulting mixture by controlling shear rate of a rod type agitator.
TABLE 1 ______________________________________ Various approaches for die-casting-like SSM forming processes Feed Conditions in the Process material barrel Comparisons ______________________________________ Rheocasting SSM pro- Constant 2-step (requires (Flemings '544) duced from temperature slurry a separate No shear producer) slurry pro- ducer Thixocasting Partially Constant 2-step (re- (Flemings 1976) remelted temperature quires SSM SSM slugs No shear billets or ingots) Thixomolding Metal Heating/shear 1-step (Bradley '589) pellets Rheomolding Liquid Cooling/shear 1-step (this invention) metal ______________________________________
Although the concept of semi-solid processing seems promising, the major problem remains as how the slurry producing and forming processes can be carded out efficiently. The possibility of getting premature freezing in the mold is higher due to the high solid fraction and high thermal conductivity of the slurry since the viscosity of a semi-solid metal is highly temperature-sensitive, the process control and mold design of SSM die casting are expected to be more difficult than in the conventional die-casting process. In this regard, numerical predictions have been proven to be very helpful for cost and time reduction for plastic injection molding. Nevertheless, due to the coupled non-linearity of the moving free surface and the inertial effects of SSM, only very limited experimental results and numerical procedures are available for the flow analysis.